Next-generation optical information transmission systems require ultrafastness, large capacity and parallel processability. Especially, the femtosecond pulse generating technique rapidly growing in recent years are indicated to be capable of processing information at a speed faster by three figures than the conventional optical information transmission techniques. In putting femtosecond optical communication to practical use, however, there remain problems that cannot be solved only by the femtosecond region pulse generating technique. While band problems in fiber optic transmission lines are important, the problem considered the most acute is to develop a device which can shape, modulate, demodulate and switch ultrashort light pulses at an ultrahigh speed that is commensurate with the width of a femtosecond pulse. Such an ultrafast device cannot be realized in the form of an electronic device but as an optical device designed to control light by light. Further, optimum is to use light phase modulation rather than light intensity modulation in order to make modulation unaffected by a transmission loss.
In order to realize optical phase modulated communication, a phase demodulating device is also important, which may, for example, embody by an up-conversion frequency generation method using a nonlinear optical material as proposed in a light multiplex transmitter-receiver by the present inventors (see Japan Patent Application No. 2001-268846, pages 19-21 and FIGS. 8-10). This method utilizes the up-conversion frequency generation by a nonlinear optical crystal to read phase information from femtosecond light pulses.
Significant progress in study of nonlinear optical effects in recent years has lead to development of optical materials that exhibit large nonlinear properties. However, realizing a phase demodulating device requires precisely assessing not only the magnitude of a nonlinear optical constant but also how such nonlinear properties relax in a femtosecond region. Unfortunately, the conventional methods do not allow measuring ultrafast temporal changes of a nonlinear property in a temporal resolution in a femtosecond region.
In view of the above problem in the prior art, the present invention seeks to provide a time resolved, nonlinear complex susceptibility measuring apparatus that is capable of assessing the performance of a nonlinear optical material for direct use in ultrafast optical communication techniques, namely of precisely measuring not only the magnitude of a nonlinear optical constant of an optical material but also an ultrafast change with time of a nonlinear complex susceptibility of the optical material in a femtosecond region.
Mention is here made of problems that arise in connection with a conventional time resolved, nonlinear complex susceptibility measuring apparatus. A Sagnac interferometric light path comprises a beam splitter serving as both an input and an out end, and a plurality of mirrors disposed so that a pair of split light beams from the beam splitter travel through an identical closed optical path while turning clockwise and counterclockwise, respectively, therethrough and then return to the beam splitter. If in this light path a test specimen is disposed at a position such that to reach it the two light beams have distances of travel different by an appropriate length, it is then possible to make an appropriate difference in time position between the two light beams passing the test specimen. Then, irradiating the test specimen with a light pulse within this time interval allows a reference light and a probe light to pass the test specimen before and after it is irradiated with a light pulse, respectively.
Light passing through a test specimen in which a nonlinear polarization is created is affected by its nonlinear complex susceptibility and then changes in phase and amplitude. It follows, therefore, that interference between the reference and probe lights outgoing from the Sagnac interferometric light path reflects a nonlinear complex susceptibility of such a test specimen, and measuring this interference allows determining the nonlinear complex susceptibility. Further, if the measurement is made while continuously varying the timing at which a test specimen is irradiated with a light pulse, it is then possible to determine a nonlinear complex susceptibility during the light pulse irradiation and that after the same in its relaxation state, namely time resolved, nonlinear complex susceptibilities.
FIG. 7 is a diagram illustrating the makeup of the conventional time resolved, nonlinear complex susceptibility measuring apparatus of Sagnac interferometer type. In the diagram, a Sagnac interferometer 101 has its light path made up of a beam splitter 102 and mirrors 103 and 104. A test specimen 105 is disposed near the beam splitter 102 in the light path and is irradiated with an excitation light pulse 106. A light pulse 107 supplied to the Sagnac interferometer 101 is split by the beam splitter 102 into a probe light 108 and a reference light 109 which propagate through the light path clockwise and counterclockwise as shown in the diagram, respectively.
FIG. 8 is a chart illustrating the timings at which the probe light and the reference light pass the test specimen in which time is represented by its abscissa axis. With different lengths between clockwise and counterclockwise optical paths to the test specimen 105, the reference light 109 will reach first and the probe light 108 will reach later the specimen 105. The test specimen 105 is irradiated with the excitation light pulse 106 in a time period after the reference light pulse 109 reaches and before the probe light pulse 108 reaches the test specimen 105. In this time period, it is possible to continuously change the timing of irradiation with the excitation light pulse 106. Before reaching the beam splitter 102 shown in FIG. 7, the probe light 108 passes through the test specimen 105 immediately after it is excited by the excitation light pulse 106 and therefore has its phase and amplitude varied by a change in nonlinear complex susceptibility of the test specimen 105 caused by the excitation light pulse 106. On the other hand, the reference light 109 passes through the test specimen 105 that is unexcited by the excitation light pulse 106 and therefore reaches the beam splitter 102 without having the phase change and amplitude change of the test specimen. Since the clockwise and counterclockwise running optical paths are equivalent to each other, the probe light 108 and the reference light 109 when they leave the beam splitter 102 only differ in phase and amplitude by changes produced corresponding to those in the nonlinear complex susceptibility of the test specimen 105 caused by its excitation by the excitation light pulse 106.
FIG. 9 is a diagram illustrating one conventional method of causing a probe light to interfere with a reference light. As shown in the Figure, the mirror 103 or 104 is shifted from its regular mirror position in the Sagnac interferometer to shift the clockwise optical path for the probe light 108 and the counterclockwise optical path for the reference light 109. Then, the curvatures of beam fronts of the probe light 108 and the reference light 109 may be utilized to cause them interfere and form their spatial interference fringes on the plane of a CCD camera 110 shown in FIG. 7. Alternatively, a white light source may be used to produce a light pulse 107 with which the Sagnac interferometer 101 is supplied and a spectroscope 111 may be disposed to form on the plane of the two-dimensional CCD camera 110 interference fringes for each of wavelengths dispersed.
FIG. 10 is a diagram illustrating spatial interference fringes according to the conventional time resolved, nonlinear complex susceptibility measuring apparatus of Sagnac interferometer type wherein the abscissa axis represents the position of the CCD camera. A difference in phase between the probe light and the reference light produced by irradiating the test specimen with the excitation light pulse can be found by measuring a deviation of peaks of interference fringes when such a difference in phase is produced between the probe light and the reference light by irradiating the test specimen with the excitation light pulse from peaks of interference fringes where there is no difference in phase between the probe light and the reference light. This diagram shows interference fringes when a difference in phase is produced between the probe light and the reference light by irradiating the test specimen with the excitation light pulse. As can be seen from the diagram, the interference fringes are shorter in period when the position is positive and are longer in period when the position is negative and are thus asymmetrical in period about the position of 0 mm. This phenomenon arises due to the fact that an equiphase wave surface is bent by the probe light passing through an excited test specimen, i.e., the phase of the probe light in a cross section perpendicular to its beam axis fluctuates. The measurement of a difference in phase will be made theoretically possible when the interference fringes are of an ideal sine wave, and gives rise to a large error under the influence of a difference in period of interference fringes in the conventional method wherein a phase difference is to be found from a deviation in peaks of interference fringes.
FIG. 11 has diagrams (a) and (b) illustrating results of measuring spatial interference fringes when a spectroscope is disposed to spatially disperse wavelengths in a conventional time, resolved, nonlinear complex susceptibility measuring apparatus of Sagnac interferometer type, in each of which diagrams the ordinate and abscissa axes correspond to a difference in optical path between a probe and a reference light and a light wavelength, respectively, and portions that appear white correspond to peaks of interference fringes. In FIG. 11 diagram (a) shows results of measurement when the test specimen is irradiated with no excitation light pulse and diagram (b) shows results of measurement when the test specimen is irradiated with an excitation light pulse. As shown in FIG. 11(b), there exist distortions in the interference fringes, which as in the case of FIG. 8 prevent a phase difference from being measured correctly.
Thus, a conventional time resolved, nonlinear complex susceptibility measuring apparatus of Sagnac interferometer type, in which optical paths for a probe and a reference light are shifted to form spatial interference fringes, has the problem that because of distortions it causes in the wave surface of probe light, it cannot measure a nonlinear complex susceptibility correctly.
As to the prior art to the present invention, reference is made to:                Y. Li, G. Eichmann and R. R. Alfano, “Pulsed-mode laser Sagnac interferometry with applications in nonlinear optics and optical switching”, Applied Optics, Vol. 25, No. 2, p. 209 (1986);        R. Trebino and C. Hayden, “Antiresonant-ring transient spectroscopy”, Optics Letters, Vol. 16, No. 7, p. 493 (1991);        M. C. Gabriel, N. A. Whitaker, Jr., C. W. Dirk, M. G. Kuzyk and M. Thakur, “Measurement of ultrafast optical nonlinearities using a modified Sagnac interferometer”, Optics Letters, Vol. 16, No. 17, p. 1334 (1991);        K. Misawa and T. Kobayashi, “Femtosecond Sagnac interferometer for phasespectrophy”, Optics Letters, Vol. 20, No. 14, p. 1550-1552 (1995); and        D. H. Hurley and O. B. Wright, “Detection of ultrafast phenomena by use of a modified Sagnac interferometer”, Optics Letters, Vol. 24, No. 18 (1999).        